As a result of the irregular distribution of solar radiation over the surface of the earth, a known temperature distribution arises between the tropics and subtropics on the one hand, and the polar regions on the other hand. This energy gradient is responsible for the known, prevailing large global wind systems, which transport a majority of the radiant energy absorbed by the Earth's surface in the form of wind, to regions including Europe, the Atlantic, the North Sea, etc. in the case of the northern hemisphere.
For many centuries now, mankind has been utilizing the power of the wind, a power which corresponds, on average, to an energy output of approximately 1 kilowatt per square meter of surface area. Using modern wind power stations, this energy can also be converted into the form of electrical energy and used in manifold ways.
Wind force and wind direction are subject to climatic and natural variations, however, and the wind frequently can cease, as well. An industrial society can only use wind power as a reliable energy source if it is continuously available, however.
One possibility for achieving a more continuous delivery of electrical energy to a power supply network or the like is afforded when a portion of the electrical energy thus generated can be temporarily stored in a sufficient quantity over a time period of at least hours, and possibly days. The energy from the temporary storage can then be fed into the power supply network during a lull in the wind so that a continuous delivery is ensured. The same applies to electrical energy from photovoltaic systems.
If such storage were successful on a large scale, these renewable energy sources could be used to cover base load demand better, and could one day fully replace fossil energy generation and nuclear energy.
Traditional direct storage devices for electricity, such as rechargeable batteries, are only able to store relatively small quantities of energy, have high storage losses, and are also very expensive and thus not economically usable on a large scale. The currently much-discussed chemical storage methods (e.g., electrolysis of water) and compressed air storage have a relatively poor efficiency in the recovery of the energy thus stored because of the heat losses alone. A utilization efficiency on the order of only approximately 30% is a good value for these storage types.
Consequently, according to the current state of the art, only pumped storage hydroelectric power plants (PSH) remain as a means of storing electrical energy efficiently and on a large scale. These plants can achieve a recovery efficiency of approximately 80% for the stored energy. In these systems, water is pumped from a lower storage basin to an upper storage basin—usually to artificial storage lakes—during periods when there is surplus electricity. The larger the storage basins and the greater the height difference is, the more energy can be stored. When there is a demand for electricity, the water that has been pumped upward in this way is allowed to flow through turbines back to the lower storage basin. During this process, the difference in the water's potential energy is converted into electrical energy. The power W is obtained from the product of the height difference h between the two storage basins and the water flow rate M. This simple formula applies (for a density of water of 1000 kg/m3): W (kW)=9.81·M (m3/s)·h (m). Hence, the total capacity of the energy storage system results from E (kW)=9.81·M·h·t/3600 (hours), where t is the maximum time period in hours for lowering the water level in the upper storage basin. In the case of a pumped storage power plant, therefore, phases of pumping, storage, and power generation alternate continuously with one another. The pumped storage power plants can be started up in a minimum of time, and thus react quickly to power demand. Today, there are approximately 30 pumped storage power plants in Germany, with locations in the low mountains and even high mountains because of the required difference in height; the largest plants in Germany are at Goldisthal in Thuringia (power output approximately 1 gigawatt and energy storage capacity approximately 8.5 GWh with a usable volume of approximately 12 million cubic meters) and Markersbach in Saxony (power output approximately 1 gigawatt and energy storage capacity approximately 4 GWh). Taken as a whole, the power output of all the pumped storage power plants in Germany totals nearly 7 gigawatts.
However, the demand for such pumped storage power plants exceeds the potential capacity that is typically present, and nearby energy storage facilities are needed primarily for the wind energy harvested offshore. Thus, expanding these capacities is an important task for the national economy (see, for example, the Energy Research Center of Niedersachsen [Energie-Forschungszentrum Niedersachsen], Goslar, http://www.efzn.de). The construction of new pumped storage power plants in the mountains and in old mines as possible expansion capacities is currently under discussion. Utilization of underground mining facilities requires a large above-ground water reservoir. The construction thereof often fails because of existing residential areas or other existing uses. Moreover, the volumes present in mining facilities are small and are distributed over long, underground distances so that high-capacity power plants can only be implemented with difficulty. Furthermore, in some cases the storage requires long power transmission distances and problematic intervention in the balance of nature. Generally speaking, the availability of suitable sites for such pumped storage power plants is limited throughout the world.
For this reason, a completely different approach for new pumped storage power plants, which at first glance may seem unrealistic, is introduced here.
German Patent Application 10 2011 013,329.1 discloses a pumped storage power plant with a pressure vessel to be sunk onto the ocean floor.
WO 2011/112561 discloses an “Offshore Energy Harvesting, Storage and Power Generation System” with energy storage and power generation units that are anchored to the sea floor.